With the continuous improvement of output power and the increasing complexity of the spaceborne high-power solid state power amplifier, the abnormal performance of products is often caused by the outgassing. In this paper, the water vapor, hydrogen and oxygen outgassinged from the dielectric substrate materials are taken as the research object. And the influences of the outgassing on the microwave characteristics are analysed. The gas composition was quantitatively tested and verified, and then the substrate was degassed by vacuum baking to reduce the content of the gas. For the first time, a supplementary test was carried out by combining product application and environmental test, and a long-term and high temperature storage test was added to simulate the actual condition. And the reduction of outgassing is at least 79.1%. Finally, the degassing effect is verified by simulation based on the test results.